1. Field of the Invention
The invention relates to an automatic image analysis method in which two images of the same object, recorded at different instants, are analyzed by
a) defining a number of image sections in an image region to be processed in one image, PA1 b) comparing each of these image sections with image sections in the other image, which occupy the same position or a position which has been shifted, within a search area, with respect to the respective image section in the one image, PA1 c) determining image sections in other image which corresponds best to the image sections in the one image, PA1 d) determining a shift between the image sections showing the best correspondence, and PA1 e) determining a transformation function approximating the shift between the individual image sections. PA1 f) a difference image section is formed from the image sections of the two images to be compared, PA1 g) a histogram is formed of the image values in the difference image section, PA1 h) the image sections in the two images which show the best correspondence being determined as the image sections whose associated difference image section has a histogram showing the narrowest frequency maximum or maxima. PA1 an image-forming device for forming at least two images of an object to be examined. PA1 means for predetermining a number of similar image sections in an image region of one image to be processed, PA1 means for comparing each of said image sections with image sections in the other image which occupy the same position as the image section in the one image, or a position which has been offset, within a search area, with respect thereto, PA1 means for determining the image section in the other image which corresponds best to the image section in the one image, PA1 means for determining the shift of the image sections thus corresponding, and PA1 means for determining a transformation function which approximates the shift of the individual image sections, is characterized in that PA1 means for comparing comprise means for forming a difference image section from respective image sections in each of the two images as well as means for forming a histogram of the image values in the difference image section, PA1 and that there are provided means for determining the image sections in the two images whose associated difference image section has a histogram showing the narrowest frequency maximum or maxima.
2. Description of the Related Art
The invention also relates to an device for carrying out the method.
One field of application is the medical diagnosis where images of a patient are formed at different instants. "Image" is to be understood to mean hereinafter a data set which characterizes a physical parameter in the pixels of a two-dimensional or three-dimensional spatial zone, for example the absorption of X-rays (in the case of X-ray images or X-ray CT images) or the strength of the transverse magnetization (in the case of magnetic resonance or MR images). The transformation function approximating the shift of the individual image sections provides quantitative information concerning the direction and extent of the deformation in the two images. Such a method enables evaluation of the changes in the vicinity of the operating zone on the basis of images taken before and after surgery, for example hip joint surgery or another surgical intervention.
Digital subtraction angiography (DSA) is a preferred field of application of the invention. Prior to injection of a contrast medium, a first image (mask image) is then recorded and after injection of the contrast medium a further image (contrast image) is recorded. The two images reproduce the same region of the patient. They deviate from one another in that the blood vessels of the patient are also reproduced in the contrast image. The difference image derived from the two images essentially shows only the blood vessels.
Because a certain period of time elapses between the recording of the contrast image and the mask image, inevitably slight movements of the patient, or deformations occur, due to respiration. The mask image and the contrast image, therefore, represent regions within the patient which have been shifted slightly relative to one another. This causes artefacts in the difference image, because the background contained in the mask image cannot be completely eliminated; the remaining background structures are undesirably superposed on the vessel structures in the difference image.
In a method of the kind set forth which is known from U.S. Pat. No. 5,048,103, first so-called "landmarks" are calculated, which landmarks are distributed as uniformally as possible across the image. An image section is then defined in which the landmark is situated at its center, and in the other image, in a search area around the pixel corresponding to the landmark, the image section is determined which corresponds best to the section around the landmark. The correspondence is determined by means of cross-correlation, the image sections with the best correspondence in the two images being determined as the sections for which the cross-correlation has a maximum. The cross-correlation however, leads to errors if there are no corresponding image sections, for example because the image section contains the image of a vessel in one image whereas no such vessel is imaged in the other image. The shift of the image section thus determined, therefore, does not correspond to the actual shift and hence the transformation function derived therefrom does not correctly describe the changes. Therefore, if the one image is transformed in conformity with the transformation function and the two image are subtracted, the artefacts in the difference image will not be eliminated.